JPH1166543A - Magnetic recording medium and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic recording medium which is fully durable-and suitable for high-density recording. SOLUTION: A non-magnetic undercoat layer containing carbon black and a binder is set on a flexible non-magnetic supporting body, on which a magnetic layer of a thickness of not lager than 0.5 μm containing sheet-like ferromagnetic particles, alumina particles, carbon black and a binder is set. The ferromagnetic particles have a specific surface area of 35 m<2> /g or larger according to BET, an average particle size of 1-50 nm and a coercive force of not smaller than 1,800 Oe. In the thus-formed magnetic recording medium, an area ratio of the alumina particles exposed at a surface of the magnetic layer is set to be 0.2-2.5%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic recording medium, and more particularly to a magnetic recording medium having a high output at a high frequency suitable for high-density recording, having excellent durability and weather resistance, and having an appropriate head cleaning characteristic. It is about.

[0002]

2. Description of the Related Art In recent years, the density of magnetic recording media has been increasing.
The recording wavelength has also become shorter. In response to this demand for higher density, a magnetic recording medium using a metal thin film for the magnetic layer has been studied. However, a coating type magnetic recording medium is superior in terms of productivity, durability, corrosiveness and the like. For this reason, studies are being made to improve the electromagnetic conversion characteristics of the coating type magnetic recording medium. In recording at a high frequency, the problems of self-demagnetization loss at the time of recording and thickness loss at the time of reproduction are increasing. In order to avoid this problem, the coating thickness must be reduced. However, simply reducing the thickness of the magnetic layer deteriorates durability and surface properties. Therefore, a magnetic recording medium having a multilayer structure in which a non-magnetic layer containing a binder and a non-magnetic powder is provided below a layer containing a magnetic powder has been proposed (Japanese Patent Application Laid-Open No. 62-159338). JP-A-62-1
No. 54225).

[0003]

However, in the prior art, sufficient durability for high-density recording cannot be obtained even with a multilayer structure. Japanese Patent Publication No. 6-73176 discloses that a magnetic layer contains an inorganic filler having a Mohs hardness of 5 or more, which is coated with an oxide, and that the presence ratio of the filler is increased near the surface to improve durability. Has been proposed. However, when the multilayer coating is performed in a wet state, if the coating thickness becomes thinner than a certain level, the inorganic filler in the magnetic layer will settle down to the lower layer, the interface will be roughened, the electromagnetic conversion characteristics will be degraded, and the roughened interface will affect the surface. And satisfactory surface properties cannot be obtained.

Japanese Patent Application Laid-Open No. 7-311934 discloses that a magnetic layer having a thickness of 0.25 μm or less is provided on a non-magnetic underlayer, and the magnetic layer contains a non-magnetic powder having a thickness larger than that of a non-magnetic underlayer. It is proposed to improve. However, this method is insufficient as a solution because a new problem that the interface and the surface are roughened and the electromagnetic conversion characteristics are degraded occurs.

Japanese Patent Publication No. 7-78868 discloses that a lubricant is contained in a non-magnetic underlayer provided on a non-magnetic support, and the lubricant concentration of the non-magnetic under-layer is adjusted to the lubricant contained in the magnetic layer. It has been proposed to improve the durability by making it higher than the concentration. However, if a large amount of the lubricant is contained, a problem of head contamination occurs, and dust adheres easily to the surface, so that a defective product generation rate during a manufacturing process and an error generation rate during use increase.

When a plate-like ferromagnetic powder is used as the magnetic powder, it is generally difficult to disperse the magnetic powder due to its very small particle size. There is a problem that the dispersion of the alumina particles progresses too much, the alumina particles settle in the magnetic layer, and the abundance on the surface decreases.

[0007]

Means for Solving the Problems The present inventors have conducted intensive studies to solve the above problems, and as a result, by specifying the area ratio of alumina particles exposed on the surface of the magnetic layer, the interface roughness, head contamination, and The present inventors have found that problems such as sex can be solved and completed the present invention. That is, the gist of the present invention is:
A non-magnetic underlayer containing carbon black and a binder is provided on a flexible non-supporting support, and a specific surface area by a BET method is 35 m ² / g or more and an average particle size is 1 to 50 n.
m, a magnetic recording medium provided with a magnetic layer having a thickness of 0.5 μm or less containing a plate-like ferromagnetic powder having a coercive force of 1800 Oe or more, alumina particles, carbon black and a binder, and was exposed on the surface of the magnetic layer. A magnetic recording medium characterized in that the area ratio of alumina particles is 0.2 to 2.5%.

[0008] Further, the gist of the present invention is to apply a non-magnetic paint containing carbon black and a binder on a non-magnetic support, to dry it, and to further contain a ferromagnetic powder, alumina particles, carbon black and a binder. A method for producing a magnetic recording medium, comprising applying a magnetic paint to be applied and providing a magnetic layer having a thickness of 0.5 μm or less, wherein the magnetic paint is made of a ferromagnetic powder, carbon black, a binder and a solvent. The present invention resides in a method for producing a magnetic recording medium, which comprises preparing a mixture, a mixture of alumina particles, a binder, and a solvent, and then combining and dispersing the obtained mixtures.

Hereinafter, the present invention will be described in detail. The magnetic recording medium of the present invention comprises a non-magnetic underlayer consisting of at least one layer provided on a flexible non-magnetic support, and a magnetic layer containing ferromagnetic powder laminated thereon. In some cases, an appropriate layer is provided on or below the magnetic layer.

The flexible non-magnetic support used in the present invention is not particularly limited. For example, polyethylene terephthalate, polyethylene-2,6-
Polyesters such as naphthalate, polyolefins such as polypropylene, cellulose triacetate, cellulose derivatives such as cellulose diacetate, aramid,
It is made of polycarbonate or the like. The form of the nonmagnetic support is usually a film, a tape, or the like. The thickness is preferably 40 μm or less. If the thickness exceeds 40 μm, the rigidity of the non-magnetic support becomes too high, the penetration of the head deteriorates, and the problem that the electromagnetic conversion characteristics deteriorate tends to occur. In order to improve the adhesion between the non-magnetic support and the magnetic layer, the non-magnetic support includes
Before forming the underlayer, for example, a corona discharge treatment or a surface treatment with a surface modifier such as an aqueous amine solution, trichloroacetic acid, or phenols may be performed.

The nonmagnetic underlayer mainly contains carbon black and a binder. As carbon black, acetylene black, black for color, furnace and the like can be used. Preferably, the specific surface area by the BET method of the carbon black is 100 to 500 m ² / g, especially 150 to 400 m ² / g, DBP oil absorption amount from 20 to 4
00 ml / 100 g. The average particle size of the primary particles of carbon black is preferably from 5 to 80 nm, particularly preferably from 10 to 50 nm. The pH of carbon black is 2 to 10,
Moisture content is 0.1-10%, tap density is 0.1-1g /
Each is preferably ml. Specific examples include BLACKPEARLS 2000, 1000, 900, 800,
VULCAN XC-72, RAVEN 880 from Columbian Carbon
0, 8000, 7000, # 3750B, # 3750, # 3250 manufactured by Mitsubishi Chemical Corporation
B, # 3250, # 950, # 850B, # 650B, # 45, # 40, # 5, MA
-77, MA-7 and the like. These carbon blacks can be used alone or in combination. Further, the surface of carbon black may be treated with a dispersant or the like, or a part of the surface may be graphitized.

The binder used for the nonmagnetic underlayer is excellent in adhesion to the support and abrasion resistance, has a glass transition point of -100 to 150 ° C., and a number average molecular weight of 1,000 to 150.
It is preferable to use one of about 000. Examples of commonly used resins include, for example, polyurethane resins, polyester resins, cellulose derivatives such as cellulose acetate butyrate, cellulose diacetate and nitrocellulose, vinyl chloride-vinyl acetate copolymers, and vinyl chloride-vinylidene chloride copolymers. Coalesce, vinyl chloride resins such as vinyl chloride-acrylic copolymers, various synthetic rubbers such as styrene-butadiene copolymers, epoxy resins, phenoxy resins, and the like. These may be used alone or in combination of two or more. Can be used. The binder is preferably used such that the content in the nonmagnetic underlayer is 2 to 50% by weight, particularly 5 to 35% by weight.

It is preferable that a three-dimensional network structure is formed in the nonmagnetic underlayer by including a low molecular weight polyisocyanate compound having a plurality of isocyanate groups in the nonmagnetic underlayer. Thereby, while improving the mechanical strength, the solvent resistance to the solvent component of the upper layer can be increased. Examples of such a low molecular weight polyisocyanate compound that functions as a crosslinking agent include a trimethylolpropane adduct of tolylene diisocyanate. Such a low molecular weight polyisocyanate compound is preferably used at a ratio of 5 to 20% by weight based on the binder. If less than 5% by weight,
The effect is not sufficient in the solvent resistance, and if it exceeds 20% by weight, the binder is plasticized and peeling is likely to occur in a calendering treatment or the like. The nonmagnetic underlayer may contain a lubricant, a dispersant, and the like in addition to the above, and the amount thereof is not particularly limited as long as the invention is not hindered. Known techniques for the magnetic layer can be applied.

The magnetic layer must contain ferromagnetic powder, alumina particles, carbon black and a binder. As the magnetic powder, ferromagnetic magnetic powder such as barium ferrite, strontium ferrite, lead ferrite, and calcium ferrite can be used. Among them, in order to form a high-density magnetic recording medium, the specific surface area by the BET method is 35 m ² / g or more, and the coercive force is 1800 Oe.
The above plate-like magnetic powder is used. Further, the magnetic properties are as follows.
It is preferably 0 emu or more. The average particle diameter (DB) of the ferromagnetic powder is preferably 50 nm or less, particularly preferably 35 nm or less. Further, the plate ratio is preferably 3 to 7 in order to increase the density of the ferromagnetic powder in the magnetic layer and increase the density of magnetic recording. The ferromagnetic powder contains 5
It is preferable that the content is contained so as to be 0 to 90% by weight, particularly 70 to 80% by weight. If the proportion occupied by the ferromagnetic powder is small, the recording density is not sufficient, and if the proportion is too large, the durability of the magnetic recording medium tends to be reduced.

The alumina particles in the magnetic layer act as an abrasive, but need to be exposed on the surface of the magnetic layer so as to occupy 0.2 to 2.5% of the surface of the magnetic layer. If the ratio of the exposed alumina occupying the magnetic layer surface, that is, the area ratio of the alumina particles is less than 0.2%, the durability is remarkably deteriorated and the cleaning property of the head is deteriorated.
A head clogging problem occurs, and the electromagnetic conversion characteristics deteriorate during long-time sliding. In addition, since the dispersion of the alumina particles in the magnetic coating material forming the magnetic layer is excessively advanced, if the area ratio of the alumina particles is less than 0.2%, the interface is disturbed by the precipitated alumina particles. . On the other hand, when the area ratio of the alumina particles exceeds 2.5%, the ratio of the alumina particles desorbed from the surface is high, and the desorbed particles damage the surface of the magnetic layer. The problem of rate rise occurs.
In the present invention, the area ratio of the alumina particles is determined by observing the surface of the magnetic layer as a flat surface, observing with a scanning electron microscope (SEM) from a direction perpendicular to the surface, and taking the alumina particles occupying the surface area from this photograph. Is calculated by calculating the ratio of the area. The area occupied by the alumina particles can be easily determined by performing image processing on a microscope photograph by a computer based on the contrast.

As the alumina particles, for example, α-alumina, β-alumina, γ-alumina and the like are used.
Specific examples include AKP-20, AKP-30, and AKP-5 manufactured by Sumitomo Chemical.
0, HIT-50, HIT-100, TF-100, TF-120, T made by Toda Kogyo
F-140, Ishihara Sangyo FT-1000, FT2000, Titanium Sangyo
STT-4D, STT-30, STT-65C, S-1 and G manufactured by Nippon Chemical Industry Co., Ltd.
5, G7, etc. Among them, those having relatively high hardness are suitably used.

The average particle diameter (DA) of the alumina particles is
It is preferably from 0.4 to 0.8 μm. If the average particle diameter is less than 0.4 μm, a large amount of alumina particles must be contained in the magnetic layer in order to secure a necessary alumina area ratio, which deteriorates the electromagnetic conversion characteristics.
If it exceeds 0.8 μm, the height of the exposed portion of the alumina particles exposed on the surface of the magnetic layer becomes too high, which causes deterioration of the electromagnetic conversion characteristics. The ratio (DA / DB) to the average particle size (DB) of the plate-shaped ferromagnetic powder is preferably 5 to 30.

In the present invention, the average height of the exposed portion of the alumina particles exposed on the surface of the magnetic layer is 0.015 to 0.05.
μm is preferred. The exposed part average height is 0.01
When the thickness is less than 5 μm, the contact area between the magnetic layer portion other than the alumina particles and the head increases, and the durability is deteriorated.
The friction coefficient increases and the traveling performance deteriorates. Also, 0.05
If it exceeds μm, the alumina layer will be detached and the magnetic layer will be damaged, and the spacing between the head and the magnetic layer will increase, and the electromagnetic conversion characteristics will also deteriorate.

As the carbon black in the magnetic layer, carbon black having an average primary particle size of 70 to 400 nm, a specific surface area by BET method of 5 to 25 m ² / g, and a DBP oil absorption of 50 cc / 100 g or less is preferably used. . Carbon black acts as a solid lubricant, but by including carbon black having a large particle size and no structure in the magnetic layer as described above, and by presenting it on the surface,
The coefficient of friction is significantly reduced, and durability and running properties are improved.
Specific examples include Asahi Thermal manufactured by Asahi Carbon, Sevacarb MT-CI manufactured by Columbian Carbon, and the like.
This carbon black is preferably present in an amount of 0.5 to 10 parts per 100 parts of the ferromagnetic powder. In addition, in addition to such a carbon black having a large primary particle size, another carbon black may be used in combination as an antistatic agent as described later.

The binder used for the magnetic layer is excellent in adhesion to the nonmagnetic underlayer and abrasion resistance, and has a glass transition point of-
100-150 ° C, number average molecular weight 1000-15000
It is preferable to use one having about 0. Examples of commonly used resins include, for example, polyurethane resins, polyester resins, cellulose acetate butyrate, cellulose diacetate, cellulose derivatives such as nitrocellulose, vinyl chloride-vinyl acetate copolymers, vinyl chloride-vinylidene chloride copolymers, Examples include vinyl chloride resins such as vinyl chloride-acrylic copolymers, various synthetic rubbers such as styrene-butadiene copolymers, epoxy resins, and phenoxy resins. These may be used alone or as a mixture of two or more. can do. The binder has a content of 2 to 2 in the magnetic layer.
It is preferred to use 50% by weight, especially 5 to 25% by weight.

It is preferable that a three-dimensional network structure is formed in the magnetic layer by including a low molecular weight polyisocyanate compound having a plurality of isocyanate groups in the magnetic layer. Thereby, the mechanical strength can be improved. Such low molecular polyisocyanate
Examples of the compound include trimethylolpropane adduct of tolylene diisocyanate. Such a low molecular weight polyisocyanate compound is preferably used in a proportion of 10 to 50% by weight based on the binder.
The magnetic layer may further contain various additives other than alumina particles, such as abrasives, dispersants, lubricants, and antistatic agents.

For example, in addition to the above-mentioned alumina particles, abrasives such as silicon nitride, boron nitride, titanium oxide, silicon dioxide, tin oxide, zinc oxide, calcium carbonate, calcium sulfate, barium sulfate, molybdenum disulfide, tungsten oxide, You may use combining silicon carbide, chromium oxide, etc.

As the dispersant, it is preferable to use a dispersant having a phosphate group such as polyether phosphate and polyoxyethylene alkylphenyl phosphate. Examples of such a dispersant having a phosphate group include phosphatidylcholine (lecithin), R
E-610 (manufactured by Toho Chemical), PW-36 (manufactured by Kusumoto Kasei)
Etc. In addition, as a dispersant, in addition to those having a phosphate ester group, capric acid, lauric acid,
12 carbon atoms such as myristic acid, oleic acid and linoleic acid
To 18 fatty acids, metal soaps of alkali metal or alkaline earth metal salts of the fatty acids, and the like.
The dispersant is contained in the magnetic layer in an amount of 0.1 to 10% by weight, particularly 1 to 5% by weight.
Preferably, it is present in the range of weight%.

As the lubricant, for example, a fatty acid ester lubricant is preferably present in an amount of 3.0 to 10.0% by weight based on the ferromagnetic powder. 2. The amount of lubricant is 3.
If it is less than 0% by weight, the durability tends to be insufficient,
In addition, the viscosity of the magnetic paint may increase, and the dispersibility and workability may decrease. On the other hand, when the content exceeds 10% by weight, the head is easily stained with the lubricant. A lubricant other than the fatty acid ester may be added. Examples of the aliphatic lubricant include fatty acids, fatty acid metal salts, fatty acid amides, and aliphatic alcohols. Fatty acids include, for example, oleic acid, lauric acid, myristic acid, palmitic acid,
Examples thereof include stearic acid and behenic acid, and the amount of the fatty acid is usually 0.1 to 10% by weight, preferably 1 to 5% by weight based on the ferromagnetic powder. If the fatty acid content is less than 0.1% by weight, the running property tends to decrease, and if the fatty acid content exceeds 5% by weight, the durability and the output tend to deteriorate.

As an antistatic agent, carbon black,
Conductive metals and their compounds, natural surfactants such as saponins, nonionic surfactants such as alkylene oxides and glycerins, cationic interfaces such as higher alkylamines, quaternary ammonium salts, pyridinium salts and other heterocyclic salts Activators, anionic surfactants containing acidic groups such as carboxylic acid groups, sulfonic acid groups, phosphoric acid groups, sulfate groups, and phosphate groups, amino acids, aminosulfonic acids, and sulfuric acid or phosphate esters of amino alcohols, etc. And the like are used. Incidentally, these surfactants may be used as a mixture.

As carbon black used as an antistatic agent, acetylene black, black for color,
A furnace or the like can be used. Specific examples include BLACKPEARLS 2000, 1000, 900, 800,
VULCAN XC-72, RAVEN 880 from Columbian Carbon
0, 8000, 7000, # 3750B, # 3750, # 3250 manufactured by Mitsubishi Chemical Corporation
B, # 3250, # 950, # 850B, # 650B, # 45, # 40, # 5, M
A-77, MA-7 and the like. These carbon blacks can be used alone or in combination. The surface of the carbon black may be treated with a dispersant or the like, or a part of the surface may be graphitized. As the conductive metal compound, tin oxide, indium tin oxide, or the like can be used. The amount of the antistatic agent used is usually in the range of 0.1 to 10% by weight in the magnetic layer. In the magnetic recording medium according to the present invention, the components constituting the above-described non-magnetic underlayer and magnetic layer are kneaded and dispersed together with an appropriate solvent to form a uniform paint, which is applied to a flexible non-magnetic support. It is manufactured by coating.

Examples of the solvent used in the present invention include ketones such as methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone; alcohols such as methanol, ethanol, propanol and isopropyl alcohol;
Esters such as methyl acetate, ethyl acetate, and butyl acetate;
Examples thereof include ethers such as diethyl ether and tetrahydrofuran, aromatic hydrocarbons such as benzene, toluene, and xylene, and aliphatic hydrocarbons such as hexane.

The preparation of the coating material for forming the nonmagnetic underlayer and the magnetic coating material for forming the magnetic layer can be carried out by a conventional method using a conventional kneading and dispersing apparatus. However, in preparing the magnetic paint for forming the magnetic layer, the ferromagnetic powder and the alumina particles are first separately mixed with a binder and a solvent, respectively, and then the resulting mixture is further mixed and further mixed with the magnetic paint. Is preferred. Usually, first, a binder and a solvent are added to and kneaded with the ferromagnetic powder, and carbon black and a solvent are added thereto to form a slurry, which is dispersed by a ball mill or the like to form a mixture. Also, a binder and a solvent are added to the alumina particles to form a slurry, which is dispersed by a ball mill or the like to form a mixture. Next, it is preferable to combine the two mixtures, add a lubricant and other auxiliaries and a solvent to a predetermined composition, and further disperse the mixture by a ball mill or the like to obtain a magnetic paint. The reason that the ferromagnetic powder and alumina are first mixed separately with a binder, a solvent, and the like is that the dispersion conditions of the alumina particles and the ferromagnetic powder are different from each other. This is because the dispersion of the alumina particles proceeds too much. Therefore, when a magnetic layer is formed using a magnetic paint prepared by combining ferromagnetic powder and alumina particles from the beginning, the alumina particles settle in the magnetic layer and the content of alumina particles on the surface of the magnetic layer decreases. Sometimes. Conversely, under the condition that the alumina particles are well dispersed, the dispersion of the ferromagnetic powder or carbon black becomes insufficient. A magnetic recording medium manufactured using such a magnetic paint often does not exhibit desired performance in many points such as electromagnetic conversion characteristics and durability. Since the alumina particles are easily dispersed, the kneading process is omitted in the above description. However, if desired, the alumina particles may be dispersed before kneading.

The formation of the non-magnetic underlayer and the magnetic layer can be performed by a conventional method using a conventional coating apparatus such as gravure coating, roll coating, blade coating, and extrusion coating. Preferably, the nonmagnetic underlayer is applied and then dried, and then the magnetic layer is applied. Usually, it is preferable to apply a magnetic paint after the non-magnetic underlayer is applied and then dried at 50 to 100 ° C. in a dryer until the residual solvent amount becomes 1.5 × 10 ⁻¹⁵ g / μm ³ or less.

In the present invention, the thickness of the magnetic recording layer existing on one surface of the flexible non-magnetic support is 0.5 μm as described above.
m or less. If the thickness is more than 0.5 μm, it is not suitable for high-density recording due to problems such as self-demagnetization loss and thickness loss. In the present invention, the maximum recording density on the magnetic layer is 70 kf.
It is preferably at least tpi. In high-density recording exceeding 70 kftpi, it is not preferable to use a plate-like magnetic powder because the magnetic conversion characteristics are greatly affected.

Before the magnetic layer is dried, a magnetic field such as an oblique magnetic field or an alternating magnetic field is applied to the magnetic layer to orient the magnetic layer.
The degree of orientation at this time is preferably from 0.95 to 1.05. After drying, the surface is smoothed by calendering. A calendering roll is usually made of a heat-resistant synthetic resin, but a metal roll can also be used. The processing temperature is preferably 70 to 120 ° C, and the linear pressure is 200 to
500 kg / cm is preferred. When the coating material contains a cross-linking agent such as a trimethylolpropane adduct of tolylene diisocyanate, the temperature of the coating is lowered to 50 to 70 ° C.
Curing is performed for a period of up to 160 hours to cause a crosslinking reaction between the binder and the crosslinking agent.

[0032]

The present invention will be described in more detail with reference to the following Examples, but it should not be construed that the present invention is limited to the following Examples without departing from the scope of the invention. In the examples, "parts" means "parts by weight". Example 1 In order to prepare a magnetic layer and a non-magnetic underlayer, the following material compositions were kneaded, and then dispersed by a sand mill to prepare a magnetic layer coating solution and a non-magnetic underlayer coating solution. However, the preparation of the coating solution for the magnetic layer was performed as follows. First, 30% by weight of a vinyl chloride copolymer was added to the alumina particles, and a solvent was further added to adjust the solid content concentration to 35%, followed by dispersion with a ball mill. Further, the remaining vinyl chloride copolymer and polyester polyurethane resin are added to the ferromagnetic powder, and a solvent is further added thereto to obtain a solid content of 75%.
%, And carbon black and a solvent were added to the mixture, and the mixture was dispersed with a ball mill to a solid concentration of 35%. Then, the two mixtures were combined, and tridecyl stearate, oleic acid and the remaining solvent were added to carry out a further dispersion treatment to obtain a coating solution.

Coating solution for magnetic layer Ferromagnetic powder (Ba.Fe ₁₂ O ₁₉ , σs = 56 emu / g, Hc = 1900 Oe, specific surface area by BET method = 43 m ² / g, average particle diameter 36 nm, plate ratio 3) 100 parts Vinyl chloride copolymer 7 parts Polyester polyurethane resin 3 parts α-alumina (average particle diameter 0.5 μm) 10 parts Carbon black 3 parts (average primary particle diameter 25 nm, specific surface area by BET method = 130 m ² / g , DBP oil absorption = 65ml / 100g) Carbon black 1 part (average primary particle size 350nm, specific surface area by BET method = 8m ² / g, DBP oil absorption = 7ml / 100g) Tridecyl stearate 3 parts Oleic acid 1 part Methyl ethyl ketone 160 parts Cyclohexanone 160 parts

Non-magnetic underlayer coating solution carbon black 100 parts (average primary particle diameter 24 nm, specific surface area by BET method = 138 m ² / g, DBP oil absorption = 60 ml / 100 g) Polyester polyurethane resin 20 parts Methyl ethyl ketone 200 parts Cyclohexanone 200 Department

To the coating liquid obtained above, 5 parts of a trimethylolpropane adduct of tolylene diisocyanate (AD30, manufactured by Mitsubishi Chemical Corporation) was added as a crosslinking agent.
The solution was filtered using a filter having an average pore size of 1 μm. A coating solution for a nonmagnetic underlayer is applied on a polyethylene terephthalate film having a thickness of 64 μm by an extrusion method so as to have a dry thickness of 0.6 μm. After the nonmagnetic underlayer was sufficiently dried, a coating material for a magnetic layer was applied thereon by an extrusion method so as to have a dry thickness of 0.15 μm and dried. After this, 100 ° C, 30
After calendering at 0 kg / cm, it was punched into a disk. The disc was kept at 60 ° C. for 72 hours for curing.

Example 2 A disk was prepared in the same manner as in Example 1 except that the amount of α-alumina was changed to 15 parts. Example 3 A disk was produced in the same manner as in Example 1 except that α-alumina having an average particle size of 0.7 μm was used as α-alumina.

Example 4 A disk was prepared in the same manner as in Example 1 except that 15 parts by weight of α-alumina having an average particle size of 0.1 μm was used as α-alumina, and the dry thickness of the magnetic layer was 0.1 μm. Produced. Comparative Example 1 A disk was produced in the same manner as in Example 1 except that the amount of α-alumina was changed to 3 parts.

Comparative Example 2 A disk was produced in the same manner as in Example 1 except that the amount of α-alumina was changed to 3 parts and the dry thickness of the magnetic layer was changed to 0.7 μm. Comparative Example 3: A disk was produced in the same manner as in Example 1 except that the amount of α-alumina was changed to 20 parts.

Comparative Example 4: In preparing the coating solution for the magnetic layer, the ferromagnetic powder and α-alumina were not separately mixed.
A disk was prepared in the same manner as in Example 1 except that all components were mixed and kneaded together from the beginning. Comparative Example 5 A disk was produced in the same manner as in Example 1 except that the following magnetic powder was used as the magnetic powder. Ba ・ Fe ₁₂ O _19: σs = 50 emu / g, Hc = 860 Oe, specific surface area by BET method = 30 m ² / g, average particle size 52 nm, plate ratio 3.1 Example 1 obtained as above 4, Comparative Examples 1 to 5
With respect to the nine types of magnetic recording media, the area ratio of the alumina particles, the average height of the exposed portions of the alumina particles, and the electromagnetic conversion characteristics were measured, and the durability and head contamination were evaluated. The results are shown in Tables 1 and 2.

[0040]

[Table 1]

[0041]

[Table 2]

The measurement and evaluation were performed as follows.
Area ratio of alumina particles: The surface of the magnetic disk was photographed with a scanning electron microscope S-4100 manufactured by Hitachi, Ltd. at an applied voltage of 20 kV and a magnification of 1500 times, and the obtained image was image-processed by a computer. Was. Average height of exposed part of alumina particles; ES made by Elionix
It measured using A-3000. Electromagnetic conversion characteristics; the disk is rotated at 1080 rpm,
Recording density of 70 kft using ferrite MIG head
pi, and the output waveform at this time was extracted.
The output value was expressed as a ratio (%) to the output voltage value of Example 1 as 100.

Durability: In an environment of 25 ° C. and 50% RH,
The disk was rotated while the head was kept in contact with the head, and the contact portion of the head was visually observed. The evaluation was performed according to the following criteria. × Scratches are generated at 20,000,000 pass or less. ○ Scratches are generated at 20 to 40 million passes.傷 No scratches occurred even after 40 million pass. Head dirt: The disk was rotated while the head was kept in contact with the head in an environment of 25 ° C. and 50% RH, and the contact portion of the head was visually observed. The evaluation was performed according to the following criteria. × At 20 million pass or less, deposits are seen on the head. △ At 20 to 40 million pass, deposits are seen on the head. Alternatively, there is a deposit on the head even if there is no scratch after 40 million passes. ○ There is no deposit on the head even after 40 million passes. As is clear from Tables 1 and 2, the magnetic recording medium of the present invention has excellent durability and excellent electromagnetic conversion characteristics.

[0044]

According to the magnetic recording medium of the present invention, there is no decrease in electromagnetic conversion characteristics due to interface roughness and surface roughness.
It is suitable for high-density recording of ftpi or more.

Claims (7)

[Claims] 1. A non-magnetic underlayer containing carbon black and a binder is provided on a flexible non-magnetic support, and B
Specific surface area by ET method is 35 m ² / g or more, average particle size is 1 to 50 nm, coercive force is 1800 Oe or more, and contains plate-like ferromagnetic powder, alumina particles, carbon black and a binder having a thickness of 0.5 μm or less. A magnetic recording medium provided with a magnetic layer, wherein the area ratio of alumina particles exposed on the surface of the magnetic layer is 0.2 to 2.5%. 2. The magnetic recording medium according to claim 1, wherein the average height of the exposed portion of the alumina particles exposed on the surface of the magnetic layer is 0.015 to 0.05 μm. 3. An alumina particle having an average particle diameter (DA) of 0.5.
3. The magnetic recording according to claim 1, wherein the magnetic recording medium has a thickness of 4 to 0.8 μm and a ratio (DA / DB) to an average particle diameter (DB) of the plate-like ferromagnetic powder is 5 to 30. 4. Medium. 4. A maximum recording density of 70 kftpi (k flux
The magnetic recording medium according to claim 1, wherein the magnetic recording medium has a transition per inch or more. 5. The carbon black in the magnetic layer has an average primary particle size of 70 to 400 nm, a specific surface area by BET method of 5 to 25 m ² / g, and a DBP oil absorption of 50 cc / 100 g or less. The magnetic recording medium according to claim 1. 6. The method for producing a magnetic recording medium according to claim 1, wherein a non-magnetic paint containing carbon black and a binder is applied on a flexible non-magnetic support, and then dried. To form a non-magnetic underlayer, on which a specific surface area by the BET method is 35 m ² / g or more and an average particle size is 1 to 5
0 nm, a plate-shaped ferromagnetic powder having a coercive force of 1800 Oe or more,
A method for producing a magnetic recording medium, comprising applying a magnetic paint containing alumina particles, carbon black and a binder to form a magnetic layer having a thickness of 0.5 μm or less. 7. A magnetic paint, comprising: a mixture obtained by kneading and dispersing a plate-like ferromagnetic powder, carbon black, a binder, and a solvent;
The magnetic recording medium according to claim 6, characterized in that the mixture is obtained by kneading and dispersing alumina particles, a binder, and a solvent, respectively, and then combining and mixing the mixtures. Production method.